1. Field of the Present Invention
This invention relates to the field of flexographic printing.
2. Background Art
A large number of consumer products are packaged in what is referred to as "flexible packaging". In flexible packaging, indicia such as company logos, labels, product information etc, is printed on polypropylene, polyethylene, foil, paper film, paper, or other non-rigid material is processed through film presses known as "flexographic" presses. A flexographic press consists of a central impression cylinder which is driven by a drive mechanism of the press. A printing cylinder is disposed adjacent the impression cylinder. One or more printing plates may be mounted on the printing cylinder with each plate having an image formed thereon for transfer to the film.
During printing, the flexographic plates are inked by way of an anilox roller which is in turn inked from an inking roller. A printing film is threaded between the impression cylinder and the printing cylinder. As the printing cylinder rotates, the image of each flexographic plate is transferred to the printing film.
Many packaged goods firms desire to employ promotions as part of the packaging of a product. For, examples, discount coupons may be printed as part of the package to encourage repeat business. In other cases, a game may be integrated into the packaging to promote the purchase of the product. One method of incorporating such games into flexible packaging utilizes the printing of quasi random number tables as part of a lottery type game and is described in U.S. Pat. Nos. 4,541,333 and 4,601,239 and in the U.S. patent application Ser. Nos. 133,666; 142,155; 160,267; and, 164,579.
In certain embodiments of the above referenced patents and patent applications, printing stations utilizing a flexographic belt arrangement for transferring images are described. An endless belt is entrained around the printing cylinder and also around an idle roller spaced from the printing cylinder. In some embodiments a tension roller may also be provided to tension the belt about the printing and idle rollers.
The belt is typically a non-elastic nylon belt having internal teeth on the inner surface of the belt which are engaged with gear teeth on the printing cylinder to keep the belt always in register with the motion of the printing cylinder. The outer surface of the belt is provided with a plurality of flexographic printing plates which are adhered to the nylon belt at a predetermined basic pitch value or gradient.
Another type of promotion which is often desired to be used in flexible packaging is the printing of the likeness of a popular character, celebrity or athlete onto the package, making it a collectible item and thereby encouraging the purchase of the product. In such promotions, it is desired to print a plurality of different likenesses on the product so that a consumer is encouraged to purchase multiple units of the product. Such a promotion requires the manufacturer to be able to print a changing image on each package. However, in the prior art methods of flexographic printing, the printing of a changing image is not possible.
One prior art method of printing a changing image is to make a number of different printing "runs" in which a different character is printed during each run. However, this method has the disadvantage of requiring a large amount of downtime while plates on the printing cylinder are changed. Another method comprises providing a custom printing cylinder with a number of plates mounted thereon, each representing a different image. However, this requires that the gradients of the plates be the same size as the repeat dimension of the package being printed. As a result, the printing cylinder would must be very large to hold even a small number of image plates. In many applications, the available space for a printing cylinder is fixed, limiting the number of different images that could be printed using the prior art method. Also, if the number of desired images were changed, a new printing cylinder must be provided. This adds to the expense of the system.
Therefore, it is an object of the present invention to provide a method of printing a changing image in a flexographic printing environment.
It is another object of the present invention to provide method and apparatus for printing of a single plate image at a time which is gradient independent.
It is yet another object of the present invention to provide a method and apparatus for inking a single plate image at a time which is gradient independent.
It is still another object of the present invention to provide a method and apparatus for printing changing images in two or more colors in registration.